AFTER Labor Day, Nantucket Island seems to breathe a deep sigh as 35,000 ``summer people'' depart by ferry, plane, or yacht for their mainland habitats. Like a wave that rolls up, up, up onto the sand, scattering its pebbles, broken shells, and seaweed, so the island sweeps to a crescendo at summer's end, hesitates briefly, and recedes languidly into autumn's golden serenity.
Whoever would experience this historic island's special charm and its finest weather can ferry the 25 miles across Nantucket Sound from Cape Cod, or fly over from New York; Providence, R.I.; or Boston.
Late vacationers arrive with the migrating ducks, geese, and swans that rest and feed at the island's freshwater ponds and brackish marshes. Beach buffs share miles of white sand with only sandpipers, herring gulls, and isolated rubber-booted surf casters.
You don't stand in line for the theater, the whaling museum, tennis courts, golf courses, boat rentals, or saddle horses. Restaurant owners, brusque with summer's crowds, become downright gallant in September. And there's sure to be room at the inn.
If you arrive without a reservation, the Nantucket Information Bureau, a few blocks from the ferry landing or a short taxi ride in from the airport, will provide a directory of accommodations. The whole range from splurges to bargains includes, among many others, the splendidly restored 1845 Jared Coffin House, the elegant 112-unit Harbor House and Cottages, the large Beachside Motel, and dozens of cozy bed and breakfast inns like the Wood Box (an authentic 1709 treasure), Chestnut House, Ship's, or Fair Winds. Rooms for two range from $45 to $235 a night.
Some shopkeepers, exhausted by the hectic summer, may lock their doors, but in general September's end-of-season sales in handcrafts, sportswear, and antiques, along with gift shops and galleries, challenge any browser's willpower. Auctions, and sport-fishing expeditions are best in fall, too.
Nantucket is a paradise for walkers and bicyclers, so park your car at the Hyannis or Woods Hole ferry wharf on the mainland. Debark two hours later and rent a three- or 10-speed bike. Some have baskets for carrying youngsters, tennis rackets, or perhaps your own harvest of plump purple beach plums, wild grapes, or seashells and driftwood.
In my basket I carry a slim guide, ``The Whole Island Catalogue,'' plus a crusty round loaf of Portuguese bread from the Nantucket Bake Shop -- and maybe a carton of fried clams from the Orange Street Fish Market. You'll find fruit stands at farms along the country roads.
You may choose to pedal east on the seven flat miles of bike path to Siasconset. Built in the 18th century as a cod-fishing camp, the village is a colony of tiny gray-shingled cottages jumbled together under blankets of climbing roses on a bluff facing out across the Atlantic. At the town pump, a compass points to ``Spain 3,000 miles, Paris 3,746, Bermuda 690.'' You can celebrate this most easterly village with an ice cream cone at the Book Store.
Northward along this bluff toward Sankaty Light, workmen are boarding up grand summer houses. The footpath ascends gradually past magnificent gardens for a mile and a half to Sankaty Head, a prime lookout, one of the few high spots on this 15-mile-long super-sandbar.
Six miles of gently rolling moorland west of Nantucket town is the sand spit Madaket, made up of creeks and a boat harbor on the sound side, a haven for surf fishing on the oceanside.
Directly south of town, a scant four-mile ride, is Surfside, a favorite beach for picnics. The Gulf Stream keeps Nantucket's waters surprisingly warm for early fall swimming. On this South Shore, waves roll in from tropical storms and crash onto broad, deserted beaches.
Nor'easter winds also blow in storms. In the calm aftermath of these, days can be as gentle and warm as summer, but the atmosphere is of such crystalline clarity that from a clifftop on Nantucket Sound you can almost count the bent cedars over on Cape Cod.
September and October are choice months for leisurely explorations on foot, too. One can stroll in the earlier dusk up cobbled Main Street as the lights come on in white-pillared mansions of former whaling captains. ``Widow's walks'' crown the rooftops. From these balustraded platforms in the mid-1800s, brigs and barkentines were sighted homeward bound from three- and four-year whaling voyages.
Daytime strolls can explore the hedgerowed lanes between some of Nantucket's 200- to 300-year-old houses (1,600 identified for historic preservation). One can trace the architectural and social history in five of these open daily to the public: Jethro Coffin (1686), Nathaniel Macy (1745), 1800 House, Hadwen House (1845), and Greater Light (1929).
Or you might walk up to the Old Mill, which began grinding corn in 1746 and, when the wind is blowing, still supplies cornmeal of superior quality. Atop Mill Hill there's a view of the splendor of the moors, which roll out like a lumpy carpet the length of the island. In autumn, this wild heath is an iridescent carpet of color, from violet heather, through the wine reds of sumac and cranberry, to the ocher of seaside goldenrod and the smoldering glow of dried summer grasses.
My favorite walk begins in early morning with blueberry pancakes at the Downyflake, next to the 1772 Rotch warehouse at the head of the wharves. When a heavy fog envelops the island, it's easy to imagine scores of tall-masted sailing ships lying at anchor in the harbor.
Brant Point's beacon revolves, piercing the gloom. The soft, wet air is permeated with the tang of tar, bait, and kelp. Missing only is the odor of sperm oil being transformed into candles. Before the Revolutionary War, Colonial craftsmen here supplied illumination for half the capitals of the civilized world.
The silence is punctured only by a melancholy foghorn, the cry of a gull, the muffled clang of the bell buoy beyond the channel entrance, creaking piles, and the muted slap of dripping lines against the gray planks of the dock.
With a July or August vacation, one may miss the allure of Nantucket, the easy surrender to a unique island in sea and time. But one can savor the tranquillity of autumn, recapture the Old World. Each fall artists and retirees succumb to the attraction and join the 5,700 who call this historic town home all year round.
But we ``off islanders'' stand on the departing ferry's stern deck tossing crackers to swooping gulls until the ``Little Gray Lady,'' as Nantucket is affectionately called, dissolves into the mist. Practical information
If you send a stamped, business-size envelope to the Nantucket Information Bureau, 25 Federal Street, Nantucket, Mass. 02554, you'll receive a free brochure describing lodgings, restaurants, sports, shops, and ferry and plane schedules. For those who prefer booking ahead, phone Nantucket Accommodations Service (617) 228-9559.